Thrombin is among the most potent of the physiological agents to induce platelet aggregation which occurs during thrombosis and hemostasis. Previously we identified a glycoprotein which is specifically cleaved by thrombin during thrombin-induced aggregation. We have now isolated this glycoprotein to apparent homogeneity and have raised an antibody against this component. In the coming year, we proposed to characterize the molecular properties of this glycoprotein and determine if it is the physiological receptor for thrombin. During this past year, we have isolated an aggregation complex consisting of glycoproteins IIb and III and actin filaments, providing direct evidence that these glycoproteins are aggregation sites on the membrane. In the coming year, we will test the hypothesis that these glycoproteins are aggregation sites on the membrane surface.